It is known that the temperature of a gas increases due to compression and that the compressed gas must be cooled before it can be supplied to a consumer network in order to prevent damage to consumers.
To this end an ‘aftercooler’ is generally used that is connected to a cooling circuit with water that flows through the aftercooler, or use is made of the surrounding air that is blown through the aftercooler.
With multistage compressors with two or more compressor elements that are connected together in series, intercoolers are also used to cool the compressed gas coming from a previous compressor element before being drawn in by a subsequent downstream compressor element, as it is known that the efficiency of a compressor element is favourably influenced by lower temperatures of the gas to be compressed at the inlet of the compressor element concerned.
In this way a lot of heat energy is lost due to heating of the coolant that is transferred to the environment as hot water or hot air.
In order to recover a proportion of this lost heat energy and to convert it into usable energy, it is known to provide such a compressor installation with a heat recovery circuit in the form of a closed circuit that is known by the name of Organic Rankine Circuit and which is provided with a pump to enable a working medium to circulate in the circuit, successively through:                one or more evaporators that act as a cooler for the compressed gas and in which the liquid working medium coming from the pump is converted into high pressure vapour due to heating by the heat of compression of the compressed gas;        an expander in the form of a turbine or similar with a rotor, a piston or similar that is driven by the heated vapour and as such ensures the conversion into mechanical energy that can be used to drive a generator or similar; and        one or more condensers that are connected to a cooling circuit of a coolant, for example water or air, to enable condensation of the vapour of the working medium into liquid that can be pumped around again by the pump for a subsequent cycle in which the working medium again undergoes a change of phase from liquid to vapour and back from vapour to liquid.        
In this way the heat of compression of the compressed gas can be converted in a known way into another usable energy form on the shaft of the turbine or similar and at the same time the compressed gas can be cooled by making use of this heat recovery circuit.
A disadvantage of this method using a Rankine heat recovery circuit is that the compressed gas is not directly cooled by the coolant but is cooled by the intervention of the Rankine heat recovery circuit that is between the cooling circuit and the compressed gas to be cooled.
A disadvantage arising from this is that when the Rankine heat recovery circuit fails due to a breakdown or leakage of the working medium or similar, the evaporator cannot exert its cooling action on the compressed gas and that in this case the temperature at the inlet of the downstream compressor element and/or at the outlet of the compressor installation can become unacceptably high.
Such a method is shown, for example, in FIG. 7 of EP 2.578.817 for cooling the compressed gas originating from a multistage compressor with two compressor elements, whereby a Rankine circuit is used with two evaporators connected in parallel that are used as coolers for the compressed gas, respectively one evaporator that is used as an intercooler between the two compressor elements and one evaporator that is used as an aftercooler downstream from the second compressor element.
The aftercooler is followed by a conventional cooler that belongs to a separate cooling circuit through which a different coolant to the working medium of the Rankine circuit is guided, whereby according to the description in EP 2.578.817 this conventional cooler is intended to cool the compressed gas to a desired temperature that is based on the intended use of the compressor installation.
When the Rankine circuit fails in this compressor installation, the two evaporators lose their function as a cooler, such that the temperature of the compressed gas at the input of the second compressor element and at the output of the conventional cooler can become higher than desired for the intended use of the compressor installation, with all possible harmful consequences thereof.
A compressor device is known from EP 0.364.106 with a number of Rankine circuits to recover the heat from the compressed gas and to convert it into mechanical energy. The gas is compressed at night and stored in an underground tank to be able to be used together with an injected fuel to supply a gas turbine during the day. In this case the cooling effect of the Rankine circuits is secondary to the recovery of heat energy. Indeed, if in this case one or more Rankine circuits fail, this will have a detrimental effect on the heat recovery but will have a rather favourable effect on the power generated by the gas turbine as the turbine will then be supplied with compressed gas at a higher temperature, in contrast to the present invention where the cooling of the compressed gas is of paramount importance.